Science Quiz

[The Last Conformist]

betazed: A bit more specification, please.

 

[betazed]

let's take a point somewhere in the galaxy. Now from this point draw two lines, one each to two diametrically opposite points of earth's orbit.

If the angle subtended by these two lines in 1/60th of 1 degree (i.e one second) then the distance to that point from Earth's orbit is one parsec.

The method of calculating distances from angles is called the method of parallax. Since over here the method of parallax gives one second of arc we call it one parsec.

 

[The Last Conformist]

That lands us on about 0.11 lightyears, I'm afraid.

 

[The Last Conformist]

Hint: One second isn't 1/60 of a degree.

 

[betazed]

Originally posted by The Last Conformist
That lands us on about 0.11 lightyears, I'm afraid.

Whoops Sorry.

One arc second is one sixtieth of a minute which is in turn one 60th of a degree.

So it is 1/3600 degree instead of 1/60 degree as I mentioned earlier.

If only I could think straight!

 

[Pirate]

I held off on answering to give betazed a chance to be more specific, so now I'll go. IIRC if we have a ruler 1 lightyear long and we sweep that ruler through 1 second of arc, the distance covered by the tip of that ruler is one parsec. (I can't remember if it's a lightyear or AU, but I'm going with lightyear)

 

[Perfection]

Wouldn't that also depend on the location? becasue the angle would be exactly the same if it was collinear with the sun and the two earth points (assuming you could see through the sun ) no matter the distance.

 

[betazed]

Originally posted by Pirate
I held off on answering to give betazed a chance to be more specific, so now I'll go. IIRC if we have a ruler 1 lightyear long and we sweep that ruler through 1 second of arc, the distance covered by the tip of that ruler is one parsec. (I can't remember if it's a lightyear or AU, but I'm going with lightyear)

Eh. IMHO, You making a mistake. For such small angles. Sin theta = theta.

So the distance moved in your case will be 1 light year X 1 arc second (in radians), which is much smaller than 1 light year. So how can it be 1 parsec (since 1 parsec is greater than 1 light year).

 

[Perfection]

Originally posted by Pirate
I held off on answering to give betazed a chance to be more specific, so now I'll go. IIRC if we have a ruler 1 lightyear long and we sweep that ruler through 1 second of arc, the distance covered by the tip of that ruler is one parsec. (I can't remember if it's a lightyear or AU, but I'm going with lightyear)

That's impossible! To get it to sweep over three times the radius you'd have to about half the circle!

 

[The Last Conformist]

betazed: you just got it to 6.524 lightyears.

Pirate: Your suggestion is right in principle, but you've got the units messed up.

 

[Pirate]

Yeah, I realized I screwed up as soon as I hit "submit reply." Is that a measure of anything? I took an astronomy lab in college 8 years ago - just wanted an excuse to go up on the roof and use the telescopes the university owned.

 

[betazed]

Ach! Google time!

As usual the devil is in the details (which I always seems to miss).

What I said was essentially correct. But instead of the whole angle you take just 1/2 the angle subtended.

http://csep10.phys.utk.edu/astr162/lect/distances/parallax.html

 

[The Last Conformist]

Indeed. It's the distance at which one AU equals one arc second.

Your go:

 

[betazed]

Ok. let's try something simple again.

How does a cat manage to rotate in mid air and land on its feet without violating conservation of angular momentum?

I know we discussed this slightly yesterday. But let's hear the details.

 

[Pirate]

Oh, so if you rotate one parsec through one second of arc it's tip sweeps out one astronomical unit. What a weird unit of measure.

 

[The Last Conformist]

Basically, it first extents one set of legs, and "screws" 180 degrees. Thanks to conservation of angular momentum, almost all of this rotation goes to the body-half with non-extended legs. Then it retracts the extended legs and extends the other set, and "screws" 180 degrees again. This ends up with the cat having turned almost 180 degrees.

Sorry, my English broke down here. I hope you get the picture.

 

[betazed]

Originally posted by Pirate
Oh, so if you rotate one parsec through one second of arc it's tip sweeps out one astronomical unit. What a weird unit of measure.

yes it is weird. It is even weider because 1 AU is also not very well defined. It is the mean distance between earth and sun. How do you define the mean distance from the circumference to the one of the foci?

 

[The Last Conformist]

Pirate: That's it.

It happens to be close to the mean distance between stars in the Sun's neighbourhood, so it's pretty practical.

 

[The Last Conformist]

IIRC, the AU was defined back when the relative sizes of plantary orbits were known, but the absolute sizes only very roughly.

 

[Perfection]

Originally posted by Pirate
Oh, so if you rotate one parsec through one second of arc it's tip sweeps out one astronomical unit. What a weird unit of measure.

Actually it's much weirder, if you rotate one parsec two seconds, the distance the two points is two astronomical units.